1. Field of the Invention
This invention relates generally to control circuits for photographic printers which are capable of automatically adjusting the color content and fade characteristics of the print.
2. Background Art
Automatic additive color photographic printers have gained widespread acceptance throughout the industry. These printers generally utilize solenoid driven light valves wherein the desired color characteristics of each scene to be printed are encoded in a paper tape so that as the film is printed the desired colored corrections are automatically incorporated. These devices utilizes a spectral light source, split into red, green and blue beams, whereby light valves along the path of these various light beams control the amount of light of each particular color used to expose the final print. Conventionally these light valves utilize a pair of opposed vanes whose angular position can be adjusted by a bar-type adder unit to vary the intensity of the light beam passing aperture between them.
The bar-type adder unit includes a plurality of solenoid actuated slides that store the desired vane position signals from the light valve punched tape. These signals represent the selected intensity and hue correction factors to be applied to the slide solenoid causing the adder unit to expand linearly by an amount corresponding to the desired correction. The information in the adder unit is converted to an angular rotation of a vane memory cam and stored in this form. The angular position of the vane memory cam is transferred to the vanes on receipt of a cue signal by actuating a vane solenoid that moves a vane into the path of a particular color light beam in an amount determined by the angular position of the vane storage memory cam. Therefore in response to a cue signal, the color information stored in the memory is transferred to its respective vane, and the valve adder is cleared by an "unlatched pulse" to receive the information corresponding to the next scene to be printed. A highly successful system of this type is described in U.S. Pat. Nos. 3,454,328 to Engel, and 3,502,409 and 3,533,693 both to Balint, all three patents assigned to the assignee of the present invention.
More recently automatic photographic printers which utilize electronic rather than solenoid control have been devised to provide more accurate and rapid adjustment of the vane position. Systems of this type are disclosed in U.S. Pat. Nos. 4,045,136 and 4,108,537 both assigned to the assignee of the present invention. These devices use a programmable electronic control system for driving the light valves in accordance with programmed color balance and intensity information. However the apparatus taught in U.S. Pat. No. 4,045,136 uses the conventional light values with bar-type adders to adjust the vane position while U.S. Pat. No. 4,108,537 uses electronic control of a PLZT cell to vary the primary light beams.
In addition electronic exposure control systems for use in photographic printers which provide fade-in, fade-out, lap dissolve or other special effects by varying the printing beam intensity in response to information supplied from a tape reader have been disclosed in the art. An exemplary system of this type is described in U.S. Pat. No. 4,009,959, assigned to the assignee of the present invention.
Despite the variety of advanced systems available, the basic solenoid driven light valve systems are still extremely popular. It may be that the high cost incurred in initially purchasing these systems is partly responsible for the reluctance to switch to more advanced techniques.